Most compact carbon dioxide (CO2) sensors are based on non-(optically) dispersive infrared detection (NDIR). CO2 has a strong infrared (IR) absorption peak at a wavelength of 4.3 micrometers (μm), which is well-isolated from those of other gases in the ambient air. In the simplest type of NDIR spectrometer, an incandescent light source passes through a volume of CO2 gas, through a 4.3 μm filter, and to a mid-infrared receiver, which is typically a thermopile. See, for example, FIG. 1 in Bates et al., “Evaluating Infrared Carbon Dioxide Sensors for 21st Century Cell Culture: introducing the Thermo Scientific IR180S1 Infrared CO2 sensor,” Thermo Scientific, 2014 (downloaded Dec. 13, 2017) (5 total pages) (hereafter “Bates”), the contents of which are incorporated by reference as if fully set forth herein. The inferred CO2 concentration is directly proportional to the absorption of the 4.3 μm light. See Bates.
Drift is the largest source of inaccuracy in NDIR CO2 sensors. Drift is caused by a variety of factors, including the incandescent light source slowly burning out, the thermopile responsivities drifting, dust and dirt accumulating within the optical path, and the ambient humidity level changing. Drift is also intertwined with the size and energy-use profile of the sensor because, all other things being equal, a more accurate CO2 sensor can use a less intense light source and/or a smaller optical path length.
To correct for drift, a wavelength slightly removed from the 4.3 μm CO2 absorption peak (such as 4.0 μm) can be used as a reference. One way to measure the second wavelength is to use a dispersive filter (see, for example, FIG. 2 of Bates), which splits the incandescent light into multiple wavelengths. However, in this configuration, the signal and reference detectors can be subject to one of the detectors selectively drifting (e.g., by dust accumulating on one of them).
Thus, improved NDIR gas sensor designs would be desirable.